IB/mthca: Check validity of output parameter pointer

The mthca driver didn't check supplied pointer to functions
mthca_cmd_poll() and mthca_cmd_wait(). This caused to the following
smatch errors:

drivers/infiniband/hw/mthca/mthca_cmd.c:371 mthca_cmd_poll() error: we previously assumed 'out_param' could be null (see line 353)
drivers/infiniband/hw/mthca/mthca_cmd.c:454 mthca_cmd_wait() error: we previously assumed 'out_param' could be null (see line 432)

In reality all callers of these functions are setting out_is_imm
flag are providing pointer too. However it is better to check
again to remove smatch errors to achieve warning free subsystem.

Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>